The avatar file size limit of 6Kb is pretty strict, but that's to address the concern of many that the avatars would slow the site speed.If you need help compressing an image email me and I can help you out.

LEAFer wrote:What extensions are allowed ? And do you have some general comments/tools/techniques for compression to achieve 6kB ?

All normal extensions - like jpg, gif, png are fine.

You can just open the image you have in an image editing software (can be something as simple as Windows Paint) and resize the image to 90x90 pixels and save. Make it a little smaller if you don't get within 6K the first time (my image is 73x73).